'\" t
.TH "SD_BUS_GET_FD" "3" "" "systemd 256.7" "sd_bus_get_fd"
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.SH "NAME"
sd_bus_get_fd, sd_bus_get_events, sd_bus_get_timeout \- Get the file descriptor, I/O events and timeout to wait for from a message bus object
.SH "SYNOPSIS"
.sp
.ft B
.nf
#include <systemd/sd\-bus\&.h>
.fi
.ft
.HP \w'int\ sd_bus_get_fd('u
.BI "int sd_bus_get_fd(sd_bus\ *" "bus" ");"
.HP \w'int\ sd_bus_get_events('u
.BI "int sd_bus_get_events(sd_bus\ *" "bus" ");"
.HP \w'int\ sd_bus_get_timeout('u
.BI "int sd_bus_get_timeout(sd_bus\ *" "bus" ", uint64_t\ *" "timeout_usec" ");"
.SH "DESCRIPTION"
.PP
\fBsd_bus_get_fd()\fR
returns the file descriptor used to communicate from a message bus object\&. This descriptor can be used with
\fBpoll\fR(3)
or a similar function to wait for I/O events on the specified bus connection object\&. If the bus object was configured with the
\fBsd_bus_set_fd()\fR
function, then the
\fIinput_fd\fR
file descriptor used in that call is returned\&.
.PP
\fBsd_bus_get_events()\fR
returns the I/O events to wait for, suitable for passing to
\fBpoll()\fR
or a similar call\&. Returns a combination of
\fBPOLLIN\fR,
\fBPOLLOUT\fR, \&... events, or negative on error\&.
.PP
\fBsd_bus_get_timeout()\fR
returns the
\fIabsolute\fR
time\-out in μs, from which the relative time\-out to pass to
\fBpoll()\fR
(or a similar call) can be derived, when waiting for events on the specified bus connection\&. The returned timeout may be zero, in which case a subsequent I/O polling call should be invoked in non\-blocking mode\&. The returned timeout may be
\fBUINT64_MAX\fR
in which case the I/O polling call may block indefinitely, without any applied timeout\&. Note that the returned timeout should be considered only a maximum sleeping time\&. It is permissible (and even expected) that shorter timeouts are used by the calling program, in case other event sources are polled in the same event loop\&. Note that the returned time\-value is absolute, based of
\fBCLOCK_MONOTONIC\fR
and specified in microseconds\&. When converting this value in order to pass it as third argument to
\fBpoll()\fR
(which expects relative milliseconds), care should be taken to convert to a relative time and use a division that rounds up to ensure the I/O polling operation doesn\*(Aqt sleep for shorter than necessary, which might result in unintended busy looping (alternatively, use
\fBppoll\fR(2)
instead of plain
\fBpoll()\fR, which understands timeouts with nano\-second granularity)\&.
.PP
These three functions are useful to hook up a bus connection object with an external or manual event loop involving
\fBpoll()\fR
or a similar I/O polling call\&. Before each invocation of the I/O polling call, all three functions should be invoked: the file descriptor returned by
\fBsd_bus_get_fd()\fR
should be polled for the events indicated by
\fBsd_bus_get_events()\fR, and the I/O call should block for that up to the timeout returned by
\fBsd_bus_get_timeout()\fR\&. After each I/O polling call the bus connection needs to process incoming or outgoing data, by invoking
\fBsd_bus_process\fR(3)\&.
.PP
Note that these functions are only one of three supported ways to implement I/O event handling for bus connections\&. Alternatively use
\fBsd_bus_attach_event\fR(3)
to attach a bus connection to an
\fBsd-event\fR(3)
event loop\&. Or use
\fBsd_bus_wait\fR(3)
as a simple synchronous, blocking I/O waiting call\&.
.SH "RETURN VALUE"
.PP
On success,
\fBsd_bus_get_fd()\fR
returns the file descriptor used for communication\&. On failure, it returns a negative errno\-style error code\&.
.PP
On success,
\fBsd_bus_get_events()\fR
returns the I/O event mask to use for I/O event watching\&. On failure, it returns a negative errno\-style error code\&.
.PP
On success,
\fBsd_bus_get_timeout()\fR
returns a non\-negative integer\&. On failure, it returns a negative errno\-style error code\&.
.SS "Errors"
.PP
Returned errors may indicate the following problems:
.PP
\fB\-EINVAL\fR
.RS 4
An invalid bus object was passed\&.
.RE
.PP
\fB\-ECHILD\fR
.RS 4
The bus connection was allocated in a parent process and is being reused in a child process after
\fBfork()\fR\&.
.RE
.PP
\fB\-ENOTCONN\fR
.RS 4
The bus connection has been terminated\&.
.RE
.PP
\fB\-EPERM\fR
.RS 4
Two distinct file descriptors were passed for input and output using
\fBsd_bus_set_fd()\fR, which
\fBsd_bus_get_fd()\fR
cannot return\&.
.RE
.PP
\fB\-ENOPKG\fR
.RS 4
The bus cannot be resolved\&.
.RE
.SH "NOTES"
.PP
Functions described here are available as a shared library, which can be compiled against and linked to with the
\fBlibsystemd\fR\ \&\fBpkg-config\fR(1)
file\&.
.PP
The code described here uses
\fBgetenv\fR(3), which is declared to be not multi\-thread\-safe\&. This means that the code calling the functions described here must not call
\fBsetenv\fR(3)
from a parallel thread\&. It is recommended to only do calls to
\fBsetenv()\fR
from an early phase of the program when no other threads have been started\&.
.SH "HISTORY"
.PP
\fBsd_bus_get_fd()\fR,
\fBsd_bus_get_events()\fR, and
\fBsd_bus_get_timeout()\fR
were added in version 221\&.
.SH "SEE ALSO"
.PP
\fBsystemd\fR(1), \fBsd-bus\fR(3), \fBsd_bus_process\fR(3), \fBsd_bus_attach_event\fR(3), \fBsd_bus_wait\fR(3), \fBsd_bus_set_fd\fR(3), \fBpoll\fR(3)
